The present invention relates to gas detecting or measuring methods and devices in which a flow of electrons is generated in a vacuum enclosure from a field-emission cathode including an array of electron-emitting micropoints associated with a grid. The electrons are directed into an ionization cage containing the gas to be analyzed and generate a flow of ions which is then analyzed by a processor such as a mass spectrometer.
Electron generators including field-emission cathodes with electron-emitting micropoints were first developed a few years ago. In such devices, electrically conductive micropoints are formed on an appropriate conductive substrate and occupy cavities in an insulative layer covering the substrate, with their ends flush with a positively biased grid having openings facing each cavity. The sharp tips of the micropoints cause local amplification of the electric field, which encourages the emission of electrons at ambient temperature and enables such emission from a threshold voltage in the order of 50 to 100 volts, depending on the nature of the array of micropoints. A mass spectrometer associated with a field-emission cold cathode with micropoints is described in Document EP-A-0 884 762. A second cathode which has a thermal emission filament refines the analysis of the gases by producing two spectra for resolving ambiguities.
Of the available means for generating a flow of electrons in a vacuum enclosure, field-emission cathodes, also known as cold cathodes, have significant advantages over conventional sources in the form of a tungsten filament heated to a temperature from 1000xc2x0 C. to 2000xc2x0 C.
In particular, field-emission cathodes have very high energy efficiency because the micropoints emit electrons at ambient temperature, whereas tungsten filaments require a high input of electrical energy to heat them to a temperature at which electrons can be emitted by a thermo-electronic effect; the orders of magnitude of the powers involved are approximately 10 watts for a heated filament and approximately 0.2 watts for a field-emission cathode.
Field-emission cathodes also have the advantage of a fast response time, both at the beginning of emission of electrons and at the end of emission; it is therefore possible to de-activate them instantaneously, unlike a tungsten filament whose temperature and corresponding emissive properties decrease only slowly, because of its thermal inertia.
Field-emission cathodes also have the advantage of generating a directional electron beam, because they emit all the electrons perpendicularly to the surface of the array of micropoints, unlike a filament, which emits electrons in all directions around the filament.
The absence of heat dissipation is another advantage of field-emission cathodes, avoiding interference with surrounding temperature-sensitive electronic circuits.
Field-emission-cathodes operate correctly when the residual gas pressure in the vacuum enclosure is below approximately 10xe2x88x925 hPa. However, producing and maintaining a sufficiently low residual pressure in the vacuum enclosure requires appropriate pumping means and, most importantly, a sufficiently long pumping time. This is a drawback in gas analysis or detection applications, in which the electron generator is used in an enclosure in which a vacuum is established intermittently: it is necessary to wait for a sufficiently hard vacuum to be obtained before carrying out the analysis or measurement.
There is therefore a requirement to operate at residual gas pressures greater than 10xe2x88x925 hPa and which can be obtained in shorter times and with simpler means.
However, for a given bias voltage between the cathode and the grid, the flow of electrons produced by field-emission cathodes decreases as the residual gas pressure in the vacuum chamber increases. The increased residual gas pressure in the vacuum enclosure requires the cathode bias voltage to be increased to obtain a given flow of electrons. Accordingly, gas detector or measuring devices generally increase the grid bias voltage to compensate for a reduction in productivity, in terms of electron flow, in the presence of a high residual gas pressure. However, the service life of field-emission cathodes is found to decrease very quickly as the residual gas pressure in the vacuum enclosure increases. If the field-emission cathode operates in an atmosphere with a residual gas pressure greater than 10xe2x88x925 hPa, progressive localized deterioration is caused by breakdown (discharges between the micropoints and the grid), with a high risk of generalized breakdown and of explosion, caused by the micropoints melting.
The problem addressed by the present invention is that of designing means of reducing the risks of breakdown of field-emission cathodes used in gas detector or measuring devices for a given geometry of the array of micropoints and for a given flow of emitted electrons.
The present invention stems from the surprising observation that, for the same flow of emitted electrons, the risk of breakdown is significantly decreased if the micropoints of the field-emission cathode are heated.
This result is surprising because heating intensifies molecular motion and would appear at first sight to increase the risk of breakdown; similarly, intentional heating of the micropoints would appear at first sight to be cumulative with the heating effect of localized microdischarges.
The present invention exploits this observation to solve the problem of breakdown of field-emission cathodes operating at pressures higher than 10xe2x88x925 hPa by proposing a gas detector or measuring device including a vacuum enclosure containing an anode forming an ionization cage for generating an outflow of ions, a processor for discriminating and measuring ions in the outflow of ions, and a field-emission cathode with an array of electron-emitting micropoints associated with a grid and generating an incoming flow of electrons into the anode, the detector further including heater means for heating the micropoints to a temperature higher than ambient temperature and maintaining them at that temperature during emission of electrons.
The heater means can advantageously be adapted to heat the micropoints to a temperature greater than approximately 300xc2x0 C. and to maintain them at that temperature during emission of electrons.
Good results have been obtained by heating the micropoints to a temperature in the range approximately 300xc2x0 C. to approximately 400xc2x0 C. and maintaining them at that temperature during emission of electrons.
In an advantageous embodiment of the invention the micropoints are carried by a substrate incorporating the heater means.
For example, the heater means are resistive heating elements accommodated in the substrate near the micropoints and are adapted to be connected to an electrical power supply.
An electron generator of the above kind can function with a field-emission cathode housed in a vacuum enclosure where there is a residual gas pressure higher than 10xe2x88x925 hPa.
The processor can be a mass spectrometer, for example.
Heating the micropoints to a temperature in the range approximately 300xc2x0 C. to approximately 400xc2x0 C. preserves the same flow of electrons with a lower bias voltage, avoiding breakdown of the cathode. It has been possible in this way to achieve a residual gas pressure of 10xe2x88x924 hPa in the vacuum enclosure using the same field-emission cathode geometry.
Accordingly, the invention provides a method of detecting or measuring gases using a vacuum enclosure containing an anode forming an ionization cage for generating an outflow of ions, a processor for discriminating and measuring ions of the outflow of ions and a field-emission cathode with an array of electron-emitting micropoints associated with a grid and generating an incoming flow of electrons into the anode, wherein the micropoints are at a temperature higher than ambient temperature during emission of electrons, preferably a temperature greater than 300xc2x0 C., for example in the range approximately 300xc2x0 C. to approximately 400xc2x0 C.
An intermittent vacuum is generally produced in the enclosure in a gas detecting or measuring method of the above kind.